This application relates to refrigerant systems, wherein the compressor is a multi-stage compressor (e.g. a two-stage compressor), and wherein an intercooler and liquid/vapor injection are provided between the compression stages. The intercooler is preferably subjected to an ambient airflow and, such that the cooling in the intercooler is preferably provided by circuitry and components that are already part of the refrigerant system.
Air conditioning, heat pump and refrigeration systems provide cooling or heating of a secondary fluid, such as air, delivered into a climate-controlled environment. A typical basic air conditioning, heat pump or refrigeration system includes a compressor, an expansion device, a heat rejecting heat exchanger and a heat accepting heat exchanger. The heat rejecting heat exchanger is either a condenser for subcritical applications or a gas cooler for transcritical applications, while a heat accepting heat exchanger is typically an evaporator. The heat pumps also include a refrigerant flow reversing device, typically a four-way valve that allows for refrigerant flow reversals throughout the refrigerant system while switching between cooling and heating modes of operation.
To obtain additional capacity, enhance system efficiency and achieve higher compression ratios without exceeding the discharge temperature threshold, it is often the case that a two-stage compressor (or a three-stage compressor, in some cases) is provided in a refrigerant system. With a two-stage compressor, two separate compression members or two separate compressor units are disposed in series. Specifically, for instance, in the case of a reciprocating compressor, two separate compression members may be represented by different banks of cylinders connected in series. Refrigerant compressed by a lower stage to an intermediate pressure is delivered from a discharge outlet of this lower stage to the suction inlet of the upper stage. If the compression ratio for the compressor system is high (which is typically the case for two-stage compression systems) and/or refrigerant suction temperature is high (which is often the case for a refrigerant system equipped with a liquid-suction heat exchanger), then refrigerant discharge temperature can also become extremely high, and in many cases may exceed the limit defined by the safety or reliability considerations.
Thus, it is known in the art to provide an intercooler heat exchanger (or a so-called intercooler) between the compression stages to extend the operational envelope and/or improve system performance and reliability. In an intercooler, refrigerant flowing between the two compression stages is typically cooled by a secondary fluid. Quite often, additional components and circuitry are required to provide cooling of the refrigerant in the intercooler. As an example, a fan or pump is included to move a secondary cooling fluid from a cold temperature source to cool the refrigerant in the intercooler.
It is also known in the art to provide refrigerant liquid/vapor injection to reduce discharge temperature, extend the compressor operational envelope and improve system performance and reliability. In such refrigerant systems, at least a portion of refrigerant leaving a heat rejecting heat exchanger is partially expanded in an auxiliary expansion device to an intermediate pressure and temperature and routed to a point between the compression stages where it is mixed with the refrigerant partially compressed in a lower compression stage and to be delivered to an upper compression stage. As also known, the vapor injection circuit may include an economizer heat exchanger to provide additional cooling to the refrigerant circulating through the main circuit and thus provide additional capacity to the refrigerant system.
Recently, new generation refrigerants, such as natural refrigerants, are being utilized in refrigerant systems. One very promising refrigerant is carbon dioxide (also known as CO2 or R744). Particularly with CO2 refrigerant systems, an intercooler and refrigerant liquid/vapor injection functions become even more important, as these refrigerant systems tend to operate at high discharge temperatures due to high operating pressures, use of a liquid-suction heat exchanger, a high value of the polytropic compression exponent for the CO2 refrigerant and, in general, by the transcritical nature of the CO2 cycle. However, the additional cost of the circuitry and components associated with the intercooler and liquid/vapor injection, along with the limited benefits for prior art refrigerant systems utilizing conventional refrigerants, made the provision of an intercooler and liquid/vapor injection in the conventional refrigerant systems less practical.
Thus, it is desirable to provide an intercooler and liquid/vapor injection for a multi-stage compressor refrigerant system, and particularly for a CO2 refrigerant system, as well as a selective activation method of these components to achieve the most efficient and reliable operation of a refrigerant system over a wider spectrum of environmental conditions.